1. Field of the Invention
This invention relates to methods for inhibiting or preventing the deposition of alkaline earth scale from aqueous waste streams derived by utilizing resources derived from the earth. This invention relates particularly to methods of inhibiting or preventing the precipitation and inclusion of radioactive minerals in scale derived from waste aqueous streams, as well as preventing premature deposition of scale in the waste water disposal injection wells.
2. State of the Art
Many processes to obtain resources from the earth, for example, mining and oil production, result in the co-production of hot aqueous liquids. The most straight foreword of these processes is the production of geothermal brine to obtain energy therefrom.
Geothermal fluids contain thermal energy in vast amounts, which can be used to produce electrical power. In one process for producing electrical power, a naturally pressurized, hot substantially liquid geothermal brine is recovered, then suddenly depressurized, or, as termed herein, flashed. This cools the liquid brine while reducing its pressure and converting some of the brine to steam used to power steam turbine generators. The cooled geothermal brine is processed further and then typically injected into the ground to replenish the aquifer and prevent ground subsidence.
The hot, highly saline brine used to generate power is saturated or nearly saturated in many ions and minerals. As the brine is flashed many of these ions and minerals precipitate out. Some minerals, typically silicon-containing compounds, particularly silica and silicate-containing minerals, such as iron silicate, form small particles that precipitate slowly. The slow precipitation means that these already precipitating minerals form solids that plug the downstream fluid handling and injection apparatus. Therefore, it is greatly preferred to force these slowly precipitating solids out of the liquid before injection using a flocculent added to the brine in a clarification process. U.S. Pat. No. 4,874,529, issued to Featherstone, which Patent is hereby incorporated in full by reference, discusses the problem of slowly precipitating fine particulates and describes the solution of adding a flocculating agent to the secondary clarification zone as the solution to the problem.
However, not all minerals precipitate from the brine solution slowly. Alkaline earth salts, for example, barium sulfate, also known as barite, and calcium fluoride are among the minerals frequently found in significant concentrations in geothermal brines. Alkaline earth salts tend to precipitate fairly quickly. During the flashing step alkaline earth sulfates (for example, barium sulfate) start to precipitate and continue precipitating down-stream during further handling of the brine as it continues to cool. Barium sulfate presents an annoying problem in many geothermal systems since it starts to precipitate when the cooling brine is still as hot as 340.degree. F., a temperature produced early on in the energy production process, triggering the early precipitation of barium sulfate. Now, the precipitation of barium sulfate has been implicated in a more serious precipitation problem.
Recently, it has been noticed that the crystalline material precipitated at some geothermal sites becomes more radioactive as the site ages. The radioactivity, contained in naturally occurring radioactive materials, or NORMs, is believed to result from tapping into new subterranean formations containing trace amounts of a radioactive alkaline earth sulfate--radium sulfate--which apparently coprecipitates with the chemically similar barium sulfate. It is important to reduce the levels of NORMs in the material that precipitates during the clarification process on the processing equipment to as low a level as possible.
In view of the foregoing, it can be seen that the exemplary hot aqueous liquid, geothermal brines, contain dissolved components that often cause processing problems. Other hot aqueous liquids produced from mines and oil wells, among other sources, cause similar problems. All these hot aqueous liquids contain unwanted components that are best left in the solution for subsequent disposal by injection into the earth while also containing unwanted components that are best removed as quickly as possible from the aqueous stream to avoid scaling and contamination of the processing equipment. The NORMs, which are associated with barium sulfate, contaminate the scale formed with radioactive components. However, even though the precipitation of NORMs from the brine stream is to be avoided, the precipitation of silicon-containing components, including silica and silicates, is required for smooth process operation. Consequently, the precipitation of the silicon containing components is induced in at least one clarification vessel by the addition of a flocculating agent.